BORE, STROKE AND DISPLACEMENT

The automotive cylinder is a machined-smooth circular hole in a cast iron engine block. A fpw engine blocks are made of aluminium but this lightweight material wears quickly. Sometimes a machined steel cylinder sleew is used inside an aluminium block for light weight and long wear. In a liquid- cooled engine the quarter-inch th'ck walls of the cylinder are surrounded by a water jacket. Water mixed with coolant flowing in the jacket carries away the unused heat of combustion. The water jacket extends up over the top of the combustion chamber to handle the heat created there. Instead of a water jacket, an air-cooled engine has cooling fins around the outside of each cylinder. They dissipate engine heat directly to the air.
Operating within the cylinder is a machined aluminium alloy piston. The piston is made slightly smaller than the cylinder to prevent binding between the two. The piston rings in grooves around its upper half. The springy piston rings press outward against the cylinder walls. The bottom ring spreads lubricating oil onto the cylinder wall. The top rings keep lubricant scraped back so that it will not reach the combustion chamber. They also seal off combustion gases to keep the gases from slipping past the piston.

BORE AND STROKE

The inside diameter of the cylinder is called bore. Stroke is the distance the piston travels from bottom dead centre to top dead centre. The size of an engine cylinder is refer to in terms of the bore and stroke. These measurements are used to calculate the lp~ston displacement.

A square engine has bore and stroke of equal measurements. An over square engine has relatively long bore and small stroke. There are several reasons for having smaller stroke:

1 . Less friction loss

2. Reduced inertia and centrifugal loads on the engine bearings.

3. Reduced engine height and lower hood line.

NOTE: In the metric system, bore and stroke are given in millimetres.

There are several reasons for the popularity of the oversquare engine. With the shorter piston stroke, there is less friction loss because the piston does not move as far in the cylinder. Also the shorter stroke reduces inertia and centrifugal loads on the engine bearings. In addition, the shorter stroke permits a reduction of the engine height and thus a lower hood line.

Despite the advantages of the shorter-stroke oversquare engine, recent emphasis on reducing atmospheric pollutants in the exhaust has forced automobile manufacturers to lengthen the stroke. For example, one late-model Ford six-cylinder engine has a stroke of 3.91 inches (99.3 mm) compared with a 3.13 inch (79.5 mm) stroke for earlier years. This longer stroke, in effect, provides more burning time for better combustion.

PISTON DISPLACEMENT

Piston displacement is the volume that the piston displaces, or 'sweeps out', as it moves from BDC to TDC. You can pasture this volume as a cylinder that is the diameter of the engine cylinder, the top and bottom being the piston-head at the TDC and BDC positions. To calculate piston displacement, you use the bore D and the length of stroke L. Thus piston displacement of a 4 by 3 112 inch (101.6 by 88.9 mm) cylinder is the volume of a cylinder 4 inches (101.6 mm) in diameter and 3 1/2 inches (88.9 mm) long.

ENGINE DISPLACEMENT

Displacement indicates the working size of an engine. It is given in cubic inches cubic centimeters, or liters. Cubic- inch displacement is often abbreviated cu. in of CID (cubic inch displacement). Cubic centimeters is abbreviated simply cc. It's a metric unit. The other metric unit liter - is used more in formula racing and advertising to give a colorful racing note to an automobile engine.

To convert CID to cc, multiply by 16.39. Thus a 350 CID engine has a displacement of 5736 cc. A liter contains, for practical purposes, 1000 cc. So a 5 liter engine has a displacement of 5000 cc.

To convert cubic centimeters to cubic inches divide by 16.39. The 5000 cc, 5 liter engine contains about 305 cubic inches displacement. To convert liters to cubic inches, multiply by 61.025. Cubic inch figures are soon to be phased out in favor of the metric ones. Ford is already using both cubic- inch and liter designations on its V-8 and V-6 engines.

Definition engine displacement is defined as the volume displaced by all of the engine's piston during one complete revolution of the crankshaft. This is also referred to as swept volume.

It's easy to calculate engine displacement. Merely multiply the area of one cylinder - that is, its cross-sectional area - times the pistons stroke. If all the figures are in cubic inches. If all the figures put into the calculate are in centimeters, the resulting figure will be in cubic centimeters.


The cross section at a cylinder is simple to calculate because it's a circle. It you know the radius of the circle you can figure the area of the circle quite easily by multiplying the radius squared times pi, which is approximately 3.14. The figure pi is a commonly used constant in circular, spherical and other calculations.


Engine manufactures give cylinder bore specifications, but not cylinder radii. Cylinder bores are diameters. Turning a diameter into a radius is simple. Divide it by two. A 4- inch cylinder bore has a radius of 2 inches.


The displacement formula could be stated simply like this. Displacement = 3.14 x stroke x no. of cyls. You need not remember any of these formulas, however. Just try to understand what they are doing.


For example, if the bore is 4 inches and the stroke is 3.5 inches in an eight-cylinder engine, the displacement would be calculate as follows: 3.14 x 4/2 x 4/2 x 3.5 x 8 = 351.68 cubic inches. Or in other words.

Displacement of an engine is a measurement of its size and is equal to the number of cubic inches the piston displaces as it moves from bottom dead centre to top dead center. In other words, it is equal to the area of the piston, it also necessary to multiply by the number of cylinders.

Displacement = A x L x N

Where A is the area of the piston in square inches, L is the length of stroke in inches and N the number of cylinders. Assuming you have a six cylinder engine with a 4 in. bore and a 4 1/4 in. stroke, the procedure is to first calculate the piston area:


Area = 4 x 4 x 0.7854 = 12.56 sq. in.

Then the displacement equals = 12.56 x 4.25 x 6 = 320.28 cu. in

Thus, Engine displacement = L X N cc

Where D = diameter of cylinder in cm
L = length of stroke in cm
N = No. of cylinder